Turning the bolt instead of the nut 4

[jerry1423]

I have always been told that when assembling a nut and bolt, that the nut should be the thing doing the turning, and the bolt should be held stationary.
I have never questioned this, but I really don't know the reason why that is.
We have a clearance problem where the bolt will have to be turned and the nut held stationary. This is a very large machine, and these are M72 hex bolts.
What are the potential problems that turning the bolt, instead of the nut, can lead to?

 

[MiketheEngineer]

The reason is to get the correct torque. Turning the bolt requires more (sometimes much more) torque than just turning the nut.

 

[MikeHalloran]

IF the bolt is not carrying a shear load during assembly, it shouldn't matter.

IF you're using elongation, not torque, to evaluate bolt tension, it shouldn't matter.



Mike Halloran
Pembroke Pines, FL, USA

 

[unclesyd]

That bolt will require substantially torque for proper tightness. I assume that you have clearance for the proper tools.

A point to make is that on large fasteners the particular lubricant used makes considerable difference in the require torque.

What stress in the bolt are you hoping to achieve?

What is the material?

 

[hydtools]

It doesn't matter. Underhead or under nut friction and shank wind-up are the same.

Ted

 

[CoryPad]

If the contact diameters for the nut and bolt are similar, and the materials/coatings are similar, then the torque will be similar whether you tighten the bolt or the nut. A recent comparison test was done by Archetype Joint, with a report available here (requires free enrollment):

http://www.archetypejoint.com/comparisontest/comparison_test.cfm

 

[dvd]

Use a screw not a bolt.

 

[jerry1423]

The definition used by the Industrial Fastener Institute (IFI) is that screws are used with tapped holes and bolts are used with nuts. Obviously a standard 'bolt' can be used in a tapped hole or with a nut.

 

[dik]

You often have a hardened washer under the nut... not the bolt end.

Dik

 

[Bivisack]

Even on small fasteners, lubrication plays a major role on preload vs torque.

I was testing some 1/4-28 titanium bolts years ago only to find out the grease that the techs were using during assembly had essentially zero effect on the preload for a XX in-lb of torque, while a proper anti-seize resulted in a ~50% higher preload for the same torque.

If it is really critical, test your hardware.

 

[ivymike]

@dik: you often have hardened washers at both ends, too.

 

[dik]

Ivymike... somethimes, but often only under the nut...

Dik

 

[Ron]

ivymike...as dik noted, in building construction, washers are rarely used under the bolt head, except for DTI's.

 

[unclesyd]

One thing not mentioned above and not very important unless you tighten a large number of fasteners.
We found over the years that if we used bolts where the head was turned, we lost a number of bolts due to damage while tightening. Our main culprit was a sledge hammer mechanic spinning an impact wrench while engaging the bolt.
It is lot cheaper to replace a buggered nut instead of a bolt.

 

[tokitalk]

You are more likely able to localize and concentrate the torque at the nut/bolt thread area where you truly need it, as oppose to transmitting it through the shank of the bolt, with amplified negative consequences especially if thread friction is high.
Also, the chances of unwanted bending may be higher with bolt tightening as opposed to tightening the nut.

 

[hydtools]

tokiabi, there is shank windup no matter which end of the bolt/nut is turned.
Is tightening a screw into a tapped hole different than tightening a bolt into a nut? I don't think so.

Ted

 

[tokitalk]

Ted: the key word in my response is THREAD FRICTION. Agreed, there is some windups of the shank either way....but with higher thread friction, the windup is considerably higher if the turning is at the bolt (think of it in terms of the torsional twist, with all other factors the same, L is longer depending on what end is reacting against the applied torque). That's the way I'm looking at it, but I'm open to differing arguments.

...still on the bolt/nut issue, do you have an explanation for why bolted assembly loses all of the torsional stress but still retains most of the initial tension after the initial relieving that takes place post tightening?

 

[ivymike]

why bolted assembly...retains most of the initial tension
because the bolt has stretched significantly.

What would make the thread friction or wind-up higher for turning one end vs turning the other? Regardless of which end you turn, you need to overcome friction at the supported face of the fastener you're turning, and the friction between the bolt and nut. If you don't overcome both of those, then you don't turn the bolt w/r/t the nut (or vice versa).

 

[CoryPad]

tokiabi,

Windup is equal whether turning nut or bolt (assuming similar friction coefficient and contact area). Please see recent testing:

http://www.archetypejoint.com/comparisontest/comparison_test.cfm

or review technical documents like VDI 2230.

Regarding loss of torsional stress, it is due to microslip at the contact surfaces. You can find recent technical articles on this subject using Google Scholar or Scirus.

 

[SafetyFactorOf1]

I am with CoryPad regarding this issue. I had to look into this issue a little while ago, because an engineer had once called out a rule on a drawing specifying "Increase torque by 10% when tightening nut rather than bolt". This sort of blanket statement set off a red flag with me. My "research" basically turned up that, if there is ample clearance in the bolt hole, the only thing that would drive some sort of rule like this would be the contact conditions of the bolt and nut faces.

http://www.boltscience.com/pages/nutorbolttightening.htm

Hope this helps